The FDA issued a complete response letter for Replimune’s RP1 oncolytic virus therapy, leading to a rapid 75% stock plunge. RP1 is an immuno-oncology candidate targeting anti-PD-1 refractory melanoma with engineered viral and immunostimulatory mechanisms. Despite promising clinical results with response rates outperforming previous oncolytic viruses, the FDA rejection marks a potential turning point in cancer therapy approvals, emphasizing heightened methodological rigor. This shift may foreshadow more stringent regulatory standards in oncology drug development, altering pathways for innovative biologics.